Electrical power converter systems are used to convert fixed frequency alternating current (AC) power to variable frequency, variable amplitude power. Such systems typically include a rectifier for converting AC power to direct current (DC) power and an inverter for converting the DC power to variable frequency, variable amplitude AC power. Frequently, a DC link transfers power from the rectifier to the inverter. The DC link may be merely an inductive coupler for transiently isolating the rectifier from the inverter. The output AC power from the inverter may be pulse width modulated (PWM) or controlled by some other means to establish the desired output power level.
Inverters may be of the current stiff type or the voltage stiff type. The voltage stiff inverter is typically used in applications requiring low to medium power, generally up to 1 MW. For example, a voltage stiff inverter using PWM is used in motor drive applications as well as in voltage regulated frequency changer applications. Current stiff converters are typically used for high power applications. Typical of these applications are motors which drive rolling mills, such as those found in steel and aluminum plants, and the motors which drive the large fans and pumps used by utilities.
Switching losses in electrical power converters are related to the switching frequency of the converters. By increasing the converter switching frequency, the size and weight of the converters can be decreased because the passive components such as inductors and capacitors can be made smaller. However, the higher switching frequency will increase the losses of the switching devices.
In the past few years, various power circuit topologies have been studied in an attempt to achieve soft switching (i.e., switching at zero voltages or zero currents) in high power converters. Soft switching operation of the power devices can significantly reduce switching losses and therefore allow operation of the converter at significantly higher switching frequencies.
Various circuit topologies have been used successfully to achieve soft switching. For example, a resonant DC link circuit (as described in U.S. Pat. No. 4,730,242, entitled "Static Power Conversion Method and Apparatus Having Essentially Zero Switching Losses") employs a resonant circuit composed of an inductor and a capacitor connected to the DC power supply and the DC bus. The LC resonant circuit is excited in such a way as to set up periodic oscillation on the inverter DC line. Under appropriate control, the DC link voltage can be made to go to zero for a controlled period of time during each cycle. During the time that the DC link voltage goes to zero, the devices across the DC link can be turned on and off in a lossless manner. By eliminating device switching losses, inverter switching frequencies can be raised to above 50 kHz at power ratings of 100 kW using commercially available switching devices such as IGBTs. However, a disadvantage to such resonant DC link circuits is that the inverter switches are required to change state synchronously with the resonating DC bus. This requires discrete pulse modulation (DPM) which precludes achieving PWM control of each phase, thus resulting in sub-harmonics and a more crudely formed sine wave at the output.